<p>The phytoplankton present on the surface and up to a certain depth of the ocean modifies the ocean’s light absorption by attenuating the incoming irradiance. This light attenuation varies with region and season depending on the phytoplankton concentration and levels. In the majority of climate models, this attenuation is parameterized by a constant coefficient. This incomplete representation can have a cascading effect on the model physics. This study assesses how the phytoplankton-light feedback may affect the Marine heatwaves (MHWs) over the Tropical Indian Ocean (TIO). A fully coupled Regional Earth System Model, namely ROM, is employed. Two experiments over the CORDEX-South Asia domain from historical to future period using RCP8.5 scenarios at 0.22°x0.22° horizontal resolution were performed. In the first experiment (INDJ), a constant light attenuation coefficient (equal to 0.06&#xa0;m^− 1) was applied in the model, and in the second experiment (INDB), a space and time-varying light attenuation coefficient depending upon the phytoplankton concentration was introduced in the model to fully incorporate the phytoplankton-light feedback into the system. Hence, the difference between these two experiments is solely due to the effect of the biochemistry module on how shortwave solar radiation penetrates into the ocean. The results revealed a noticeable difference in the MHWs intensity and duration between experiments. Notably, in the INDB experiment, the First Permanent MHWs and Absolute Permanent MHWs get delayed over approximately 55% and 64% of the area of TIO, respectively. Considerable delays (two decades) in the INDB experiment were found over approximately 29% and 26% of the TIO for the First Permanent MHWs and Absolute Permanent MHWs, respectively. This study suggests that phytoplankton-light feedback can be another physical player for simulating and understanding MHWs in the climate models.</p>